Modern aircraft require removable panels and inspection doors that are part of the exterior skin surface. Mechanical fasteners used to secure these and other installations that have parts of the fastener that extend to the outside surface present concern when the fasteners are located in critical fuel vapor areas. A critical fuel vapor area is an area where fuel and air exist in a stoichiometric mixture that can be ignited by a low intensity spark. Currents from a lightning spark can be conducted from a door or panel into the bolts or directly from the lightning channel into the bolt and then thru the bolt into the mechanical fastener. The many metal-to-metal surfaces of the fasteners have contact resistances which will heat up from Joule heating when the current flows. If the level of current is large enough the heating will cause hot sparks to be driven off the fastener into the critical fuel vapor area.
Mechanical fasteners in the critical fuel vapor area are typically dome nut fasteners. Dome nuts are used where fuel leaks to the outside skin must be prevented. If the dome nut fastener is relatively large the conductive metal interfaces may carry high currents without sparking. There are situations where relatively large dome nuts are not practical or possible. A relatively small dome nut fastener will not be able to transfer the current of a lightning strike without sparking at the interfaces.
In the past, attempts have been made to use fuel sealant or adhesive as a shielding or isolation material to prevent the sparks from entering the critical fuel vapor area without success. This shielding material was normally applied as a viscous substance which after a given time cured into a hardened material. In a wide bodied aircraft there are potentially a thousand or more fasteners which could be sealed to prevent sparking. Due to the large number of fasteners involved air gaps and voids were unavoidable in normal methods of application even when seemingly large amounts of sealants were applied over the sparking areas. These air gaps and voids were found to be the source of sparking in the fuel vapor area. Visual inspection of the isolation material was not adequate to detect the air gaps and voids because the location of the sparking fastener is hidden by the opaque material giving uncertainty to the thickness of the applied isolation material. Lightning simulation testing proved that air gaps and voids could not be avoided in the normal application of the isolation material.